1. Field of the Invention
This invention relates to a shell-and-tube heat exchanger and a method for inhibiting polymerization in the shell-and-tube heat exchanger. More particularly, this invention relates to a shell-and-tube heat exchanger intended to handle an easily polymerizable substance and characterized by having no stagnating part of a process fluid on the surface thereof for contact with the process fluid. Particularly, this invention relates to a method for inhibiting polymerization in a shell-and-tube heat exchanger, characterized by using a seamless steel tube as the heat transfer tube for the shell-and-tube heat exchanger during the operation of the heat exchanger on an easily polymerizable substance. This method permits effective inhibition of the polymerization of an easily polymerizable substance which possibly occurs in a heat transfer tube.
2. Description of Related Art
The shell-and-tube heat exchanger for effecting exchange of heat extensively between two fluids, one having a high temperature and the other a low temperature, is one of the chemical machines that are extensively used in the chemical industry. The shell-and-tube heat exchanger exhibits high reliability under harsh use conditions and during a protracted continuous operation and, owing to this conspicuous merit, has been finding greater utility than any other kind of heat exchanger. In all the species of shell-and-tube heat exchanger, the tube sheet heat exchanger is characterized by containing at least one built-in tube sheet in each of the opposite terminal parts of a tubular shell thereof and interconnecting the opposed tube sheets with a multiplicity of heat transfer tubes having the opposite terminal parts thereof fixed peripherally.
In the conventional vertical tube sheet heat exchanger such as is illustrated in FIG. 1, for example, the tube side fluid (11) is introduced into the heat transfer tubes via the upper tube sheet (2a) near the shell side fluid outlet (6) and then discharged from the terminals of this heat transfer tubes (1) out of the heat exchanger via the lower tube sheet (2b) and the lower channel (5). A shell side fluid (10) is introduced into a shell (3) via an shell side fluid inlet (7) and is discharged from the shell (3) via an shell side fluid outlet (6), with the result that heat is exchanged between the tube side fluid (11) and the shell side fluid (10). The connecting parts used herein between the tube sheet (2a, 2b) and the heat transfer tubes (1) are so fixed as to allow protrusion of the heat transfer tubes (1) from the tube sheet as illustrated in FIG. 3a with the object of securing strong connection and permit easy attachment of a multiplicity of heat transfer tubes (1). The heat exchanger generates vibration because of constant inward and outward passage respectively of the tube side fluid (11) and the shell side fluid (10) and the heat transfer tubes (1) receive this vibration because of transmission of vibration from a pump and a compressor and direct pulsating flow from rotary machines. For the purpose of preventing this vibration from loosening the fixing parts of the heat transfer tubes (1) and ultimately inducing the fixing parts to leak, therefore, the strength necessary for this prevention is secured by causing the heat transfer tubes (1) to protrude from the surface of the tube sheets (2a, 2b). Further, in order to exalt the efficiency of heat transfer, it is necessary that the surface of contact with the shell side should be enlarged and the heat transfer tubes (1) should be reduced in diameter. The object of fixing stably the multiplicity of such slender heat transfer tubes (1) is easily attained by joining the heat transfer tubes (1) to the surface of the tubes sheets (2a, 2b) as protruded therefrom.
The surfaces of heat transfer in the shell-and-tube heat exchanger are gradually fouled with a deposit of extraneous substance and degraded in the efficiency of heat transfer with the elapsing of use time. The fouling requires the heat exchanger to be repaired and prevents it from allowing a protracted operation easily. This fouling occurs when the fluid contains a solid substance or a semi-solid substance. When the process fluid itself is not a solid substance but has an easily polymerizable substance as a component thereof, it produces a polymer in the shell-and-tube heat exchanger and the polymer fouls the heat exchanger. Particularly, in the case of the shell-and-tube, the polymerization generated in the heat transfer tubes (1) tends to interfere with the protracted continuous operation of process because this heat exchanger uses numerous heat transfer tubes of a minute diameter with a view to enhancing the efficiency of heat transfer. The adhesion of the polymer to the inner walls of the heat transfer tubes results in obstructing the process of continuous purification by degrading the heat transfer tubes in the efficiency of heat transfer and clogging the heat transfer tubes.
The shell-and-tube heat exchanger (condenser) which is led from the top of a distillation column, for example, causes the vapor ascending to the top of the distillation column and abounding in a low boiling component to be cooled and condensed in the heat transfer tubes. When the object of distillation is such an easily polymerizable compound as acrylic acid, it tends to polymerize in the condenser. Acrylic acid, for example, is produced by catalytic vapor phase oxidation of propylene and/or acrolein with a molecular oxygen-containing gas and the acrylic acid-containing liquid consequently formed is distilled in the distillation column. In the composition of this acrylic acid-containing liquid, the acrylic acid is unusually susceptible of polymerization because this composition contains such impurities as water, acetic acid, and acrolein.
Further, the degree of the polymerization of the easily polymerizable substance increases in accordance with the heating advances. Since the internal temperature of the distillation column in the process for purifying an easily polymerizable substance is governed by the boiling point of the easily polymerizable substance being purified and the boiling point of a solvent which is contained in conjunction with a high boiling substance to be separated and the easily polymerizable substance, it is difficult to adjust the internal temperature of the shell-and-tube heat exchange solely for the purpose of preventing the polymerization. Especially, when the shell-and-tube heat exchanger is used as a re-boiler, it tends to induce the polymerization because it has a particularly high working temperature.
For the purpose of preventing the polymerization of such an easily polymerizable substance, the practice of adding various polymerization inhibitors such as phenothiazine, hydroquinone, methoquinone, cresol, phenol, and t-butyl catechol to the purification process has been in vogue. Such a polymerization inhibitor is a high boiling substance. It is allowed to manifest a fully satisfactory effect of inhibiting the polymerization in the bottom liquid of the distillation column. Under the temperature conditions that cause the easily polymerizable substance to vaporize, however, this polymerization inhibitor is not contained in the produced vapor. In the condenser disposed at the top of the distillation column, since the vapor easily polymerizable substance does not contain the polymerization inhibitor, the composition itself assumes a state extremely susceptible of polymerization and incurs difficulty in repressing the polymerization of the easily polymerizable substance in the vapor state. When the tube sheet heat exchanger has a shell-and-tube construction, therefore, since it has such protrusions of heat transfer tubes as mentioned above on the upper tube sheet of the condenser, the polymer adheres to the protrusions. The adhesion of the polymer results in degrading the efficiency of separation, clogging the heat transfer tubes, and consequently obstructing the protracted continuous operation of the process.
It has been heretofore necessary, therefore, to give a periodic inspection to the interior of the shell-and-tube heat exchanger and deprive the shell-and-tube heat exchanger of the adhering polymer, if any, with the object of maintaining the efficiency of heat exchange at a high level and, at the same time, preventing the efficiency of separation and purification from lowering and the quality of product from deteriorating.
The present inventors have discovered that the polymerization occurring otherwise in the shell-and-tube heat exchanger can be effectively inhibited by preventing the stagnation of the process fluid possibly induced on the surfaces of contact with the process fluid such as of the surface of the tube sheet. This invention has been perfected as a result. Further, the present inventors have made a detailed study of the polymer which occurs when an easily polymerizable substance is subjected to a treatment in the shell-and-tube heat exchanger and have consequently discovered that the polymer adheres to such minute undulations as in a seam which is formed during the production of a heat transfer tubes and that the adhesion of the polymer can be prevented by eliminating these undulations.
Specifically, this invention has the object of providing the following items (1) to (3).
(1) A shell-and-tube heat exchanger for handling an easily polymerizable substance, composed of a shell having built therein near the opposite terminals thereof two tube sheets respectively furnished with an inlet and an outlet for shell side fluid (10), channels disposed one each at the opposite terminals of said shell, and a multiplicity of heat transfer tubes having the peripheries of the opposite terminal parts thereof fixed between said tube sheets, adapted to pass an easily polymerizable substance as a process fluid through said tubes and effect exchange of heat thereon, and characterized by having no part for stagnation of said process fluid on the surfaces for contact with said process fluid.
(2) In the operation of a shell-and-tube heat exchanger of Item 1 by passing there through an easily polymerizable substance as a process fluid thereby effecting change of heat thereon, a method for inhibiting polymerization in a shell-and-tube heat exchanger characterized by having said process fluid contain a polymerization inhibitor.
(3) A method for preparing (meth) acrylic acid and/or an ester thereof using at least any one of shell-and-tube heat exchanger according to Item (1).